Past designs for rotary lawnmowers commonly had the blade fixed directly to the depending drive shaft of an engine. Concerns were raised, however, regarding the danger and other disadvantages of such an arrangement. For instance, having the blade constantly spin even during starting and idling presents inherent dangers as the engine drive shaft can be bent when the blade strikes an obstruction and overloading of a direct-driven blade can stall the engine, to name a few issues. As a result, manufacturers began implementing clutch systems into lawnmower designs that allowed the blade to be disengaged without stopping the engine. One example of such a system is a blade brake and clutch system disclosed in U.S. Pat. No. 6,464,055, the disclosure of which is incorporated by reference herein in its entirety.
Because of the great difference in loading experienced by the engine between an engaged state and a disengaged state, however, some form of speed regulation device is often required to help keep the engine within desired operating ranges. Current lawnmower designs generally employ one of two configurations to control the speed of the engine. In a fixed throttle configuration, the engine speed is fixed at the maximum desirable running speed (e.g., about 3150 rpm for lawnmowers). As a result, even when the blade is disengaged from the engine, the unloading of the engine will not cause the engine to speed to exceed this predetermined limit. Alternatively, in a variable throttle configuration, the engine speed is variable in response to operation of a throttle control accessible to the operator. In this way, the operator can move this throttle control to reduce the engine speed when the blade is not engaged (e.g., when the operator is dumping the grass bag).
Each of these arrangements has advantages and drawbacks. For instance, a fixed throttle configuration does not require any additional operator input to control the engine speed, but the engine speed always races to the maximum running speed when the engine is unloaded due to governor system balancing, which can result in noisy idling. Also, these high engine speeds in the unloaded condition can result in higher fuel consumption and higher heat generation, which is particularly a concern in air-cooled engines. In variable throttle configurations, although the engine speed can be reduced in unloaded operation, this reduction generally requires the operation of a separate throttle control. Accordingly, until the operator takes the additional step to adjust the throttle control, problems with noise and high fuel consumption due to the engine racing to high speeds can likewise be problematic.
As a result, it would be desirable for a throttle system of an engine to automatically reduce the engine speed when the engine is moved into an unloaded state, such as when a blade of a lawnmower is disengaged.